1. Field of the Invention
The present invention relates to a load monitoring method for a motor-driven injection molding machine, which monitors load of a drive section using a drive motor (servomotor).
2. Description of the Relevant Art
Generally, a motor-driven injection molding machine using a servomotor at a drive section potentially involves malfunction or breakage of the servomotor due to overheat arising under certain molding conditions or under a certain environment, since the servomotor generates heat according to load current. Therefore, the motor-driven injection molding machine usually employs measures for protection of the servomotor. Specifically, a heat generation quantity of the servomotor is detected. When the detected heat generation quantity (detection value) reaches a preset stopping point, the servomotor is determined to be in an overload state and is forcibly stopped to thereby protect the servomotor.
For a motor-driven injection molding machine, stoppage of a servomotor during molding means interruption of a molding process, which may have an adverse effect on the molding process once resumed; e.g., lowering the operating ratio. Therefore, interruption of a molding process must be avoided to the extent possible. In order to meet this end, for example, Japanese Patent Application Laid-Open (kokai) No. 11(1999)-235743 proposes a method for checking previously set operating conditions. According to the proposed method, the injection molding machine is operated on a trial basis, current flowing through a servomotor is measured along with a time during which the current flows; and when the measured current and time fall within a predetermined overload-danger region set to be lower than a predetermined overload region, an operator is prompted to review the previously set operating conditions. This method avoids a problem such that, because of a relatively small variation in load caused by a disturbance and a variation in temperature of the machine during continuous operation, the actually measured current and time fall within the overload region, and the operation of the machine is interrupted.
The above-mentioned conventional method involves the following problems.
First, as a result of the previously set operating conditions being reviewed in order to provide a margin, a molding cycle may involve wasteful time, possibly lowering production efficiency and productivity. That is, since a molding cycle is typically several seconds to several minutes, a slight amount of wasteful time per molding cycle results in considerable time loss (a decrease in productivity) when the production is considered as a whole.
Second, since operating conditions are reviewed merely at the stage of trial operation, the review fails to reflect operating conditions during actual molding. Thus, the conventional method may fail to accurately set operating conditions, resulting in impaired molding quality.